The study of peroxide reactions becomes a major effort of this project in view of the ubiquity of peroxide in living cells and the new knowledge of the reaction of the unbridged form of cytochrome oxidase with peroxide. These are modelled by the reaction of the intermediates with peroxide, with hydrogen, alkyal and other peroxides, as oxidants and with a variety of heavy-atom-labelled substrates as reductants. The intermediates were studied in trapped states at low temperatures and at physiological temperatures in real time using rapid flow methods (regenerative flow). Horseradish peroxidase is of most interest because it forms two types of intermediates; as does lacto-peroxidase. In the case of those forming a single intermediate, yeast peroxidase and catalase are seriously to be considered, together with chloroperoxidase. Thus in this specific proposal, the structures of intermediates will be studied in the trapped state, and the relationship of these structures to their function will be demonstrated in room temperature rapid flow studies, with concomitant optical and epr monitoring of sample properties. Where possible, advantage will be taken of ongoing X-ray crystallography, magnetic and spin resonance methods, as well as Raman spectroscopic methods in order to ensure that conclusions based upon X-ray absorption spectroscopy are consistent with these other approaches. The results are expected to be significant in two respects: 1) the specific determination of the extent to which these compounds are appropriate models for the peroxidatic reaction of cytochrome oxidase and 2) the generalities of common mechanisms for the utilization of peroxide in biological systems.